A lanthanum hexaboride (LaB6), a cerium hexaboride (CeB6), a hafnium carbide (HfC), or the like which is crystallized or sintered is used as an electron source or emitter (hereinafter described as a cathode or a thermionic cathode) for various apparatuses (for example, scanning electron microscopes (SEM) and transmission electron microscopes (TEM)). The cathode generally has a tapered shape or truncated conical shape, and a tip thereof is cut off by a predetermined size. Further, the tip of the cathode generally has a flat or spherical shape.
However, the cathode formed of the above-described material evaporates at 1650 K to 1900 K (Kelvin) which are operating temperatures, and the size of the cathode tip shrinks. As a result, there arises a problem that the usable time (lifetime) of the cathode is limited. Further, electrons are emitted not only from the tip of the cathode but also from a tapered part. Electrons emitted from this tapered part are as many as 65% of the electrons emitted from the entire cathode. Thus, it is difficult to obtain a converged electron beam.
Accordingly, a cathode whose tapered part is covered with carbon (C) has been proposed. The evaporation rate of this carbon is very low, and its steam pressure is about 10−10 Torr at 1650 K to 1900 K as operating temperatures, which is about less than or equal to a thousandth of that of the lanthanum hexaboride (LaB6), the cerium hexaboride (CeB6), and the like. It is thus possible to extend the usable time of the cathode for about 3000 hours. Further, a converged electron beam can be easily obtained because electrons are not emitted from the tapered part covered with carbon.
However, the usable time is still limited in such a cathode. This is because evaporation or disappearance occurs partially in the cathode by chemical interaction between the cathode material (for example, the lanthanum hexaboride (LaB6), the cerium hexaboride (CeB6), or the like) and carbon. This can be seen in the edges of the emitting surface of the cathode which are in contact with the carbon coating.
In order to solve the above-described problem, there has been proposed a cathode in which a gap is provided between the cathode and the carbon covering the tapered part at the tip of the cathode. The cathode of this proposal has a structure in which a carbon layer is disposed around the cathode being a center, and the gap is provided between the cathode and the carbon layer. That is, in the cathode tip, the material (for example, the lanthanum hexaboride (LaB6), the cerium hexaboride (CeB6), or the like) of the cathode and the carbon are not in direct contact, and thus the partial evaporation or disappearance by chemical interaction can be suppressed.
However, when the width of the gap formed in the tip of the cathode differs depending on its position, an electric field distribution in the cathode tip becomes uneven, and there occurs a phenomenon such that the peak (position where a current value is highest) of an emitter current (electron beam) is displaced from the center of the cathode. As a result, an irradiation position of electron beam is affected. For example, in a mask writer, writing precision with electron beam is affected.